Barrier coatings for polymeric substrates

ABSTRACT

An exemplary embodiment provides coated polymeric substrates that have a polymeric substrate body with a coated surface. The surface coating includes more than one pair of coating layers. Each pair of coating layers includes a first applied coating layer and a second applied coating layer. In addition, an indicator, applied on top of or between coating layers, provides an indication of wear of the coating.

TECHNICAL FIELD

The embodiments described herein generally relate to barrier coatingsfor polymeric substrates and more especially to such coatings that arerelatively impervious to water absorption and that have abrasionresistance.

BACKGROUND

Plastics are finding increasing use in manufactured goods. For examplecertain automobiles have plastic body panels, aircraft have plasticinterior paneling and even exterior skin panels may be made of plasticcomposites. While plastics offer several excellent properties such aslight weight, formability, and low cost, plastics also have significantshort comings. In general, plastic surfaces are not as hard or abrasionresistant as steel surfaces, for example. Further, while some plasticsmay be transparent, glass which is much heavier and more expensiveremains the material of choice in certain critical applications such assafety glass in automobiles and in passenger aircraft windshields.

The primary cockpit windshields in commercial airplanes are typicallymade of multi-pane laminated glass. Glass is used because of itsstrength, scratch and wiper-blade abrasion resistance, and chemical andenvironmental resistance. These properties insure that good vision ismaintained through critical cockpit windshields, especially where wipersare used for rain removal. Glass has limitations, however, in terms offormability and is not always the lightest and/or least expensivematerial for windshield construction. Cockpit windshield shapes haveremained virtually the same during the last several decades because oflimitations in glass formability. Switching to polymeric materials, suchas stretched acrylic or polycarbonate, could lead to lighter, lessexpensive windshields and permit greater flexibility in terms ofwindshield shape. Reshaping of cockpit windshields would lead toimproved airplane cab aerodynamics with associated aero-drag reductionand hence improved airplane fuel efficiency. Improved cockpitaerodynamics would also lead to reduced exterior noise generation andhence might lower internal cabin noise levels. Reduced noise levels maymake airplane travel more comfortable to the flight crew and passengers.

Aircraft passenger windows, on the other hand, unlike passenger aircraftwindshields, are typically made of stretched acrylic (i.e. acrylicsubstrate) due to its light weight, flexibility and formability.However, acrylic is susceptible to particle (e.g. sand) and waterinduced erosion and crazing. Moreover, during flight, aircraft windowsare subjected to differential pressures caused by the difference inpressure between the inside and the outside of the aircraft. Thisdifferential pressure causes the windows to flex and the flexing maycause the windows or any coatings on the windows to crack over a periodof time. To avoid the fine cracks allowing the potential for chemicalsto attack the acrylic substrate and/or allowing the coating todelaminate from the acrylic substrate, the windows are replaced inroutine maintenance. This poses an additional expense and reduces thepotential in-service time of the aircraft.

It might be expected that if polymeric laminates were used aswindshields, then water absorption into such a windshield and corrosiveeffects of chemicals could lead to crazing when stress is applied to thewindshield, as may be encountered in flight. Crazing or other mechanicaldamage, such as scratches, can have a deleterious effect on operatorvision through the windshield. In addition, scratched windshield wouldhave to be replaced routinely thereby imposing additional repair costsand reducing aircraft in-service time.

Polysiloxane coatings have been used to protect polymeric substratesurfaces from chemical attack, abrasion and wear. Polysiloxane hardcoatings, applied using organosiloxane compounds, protect polymericsubstrates such as polycarbonate or acrylic from damage caused byabrasion and/or environmental exposure. These solvent-based coatings,typically a few microns thick, are applied through a dip or flow coat orspray process and then dried through a low temperature (150° F.=65.5°C.) bake. As passenger aircraft windshields are stressed under airplanepressurization these polysiloxane coatings are tailored to provide goodelongation properties, however, this limits their abrasion resistance.While currently available coatings, when applied to passenger aircraftwindshields, exhibit good optical characteristics, their scratchresistance and durability are limited based on field results. They areeither prone to cracking or they provide minimal abrasion protection.Both cracking and/or abrasion can lead to de-bonding of the polysiloxanecoating due to environmental exposure and result in scratches andcrazing of the base acrylic windshield substrate.

Accordingly, it is desirable to develop barrier coatings to protectpolymeric substrates like passenger aircraft flight deck windshieldsagainst abrasion, chemical attack, and crazing. In addition, in the caseof passenger aircraft windshields, the coatings should be reliable andhave some means to warn the flight or maintenance crews when they are nolonger effective. Further, in aircraft windshields, the coatings shouldexhibit good adhesion to the polymeric substrate, excellent wearresistance, minimal ultraviolet light-induced degradation, goodelongation/flexibility, and resistance to crazing when exposed tosulfuric acid and a host of chemicals used in aircraft cleaning andmaintenance. In addition it should have an indicator that shows thecoating condition. Furthermore, other desirable features andcharacteristics of the barrier coatings will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

BRIEF SUMMARY

An exemplary embodiment provides coated polymeric substrates that have apolymeric substrate body with a coated surface. The surface coatingincludes more than one pair of coating layers. Each pair of coatinglayers includes a first applied coating layer and a second appliedcoating layer. In addition, an indicator, applied on top of or betweencoating layers, provides indicia of wear of the coating.

In another example, an embodiment of the surface coating, after exposureto chemicals selected from de-icing fluid, jet fuel, methyl ethylketone, solvent cleaner, alkaline cleaner, hydraulic fluid anddetergent, and testing in accordance with Taber Wear Test ASTM D-1044-99for 100 cycles, exhibits a change in haze of not more than about 1%.

In a further example, an embodiment of a method of testing a polymericpassenger aircraft windshield includes the step of examining theaircraft windshield. The windshield has a coating on an external surfaceand the coating includes more than one pair of coating layers. Each pairof coating layers includes a first applied coating layer and a secondapplied coating layer. The coating also includes an indicator, appliedon top of or between coating layers that provides indicia of wear of thecoating. The method further includes the step of determining, based onthe indicator, whether the windshield requires maintenance, replacementor repair.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a schematic cross-sectional view, not to scale, of a portionof an exemplary embodiment of a barrier-coated polymeric substrate;

FIG. 2 is a process flow diagram of an exemplary process for producingbarrier-coated polymeric substrates;

FIG. 3 is a graphical representation of comparative percent change inhaze based on a Sand Erosion Test, detailed in the Examples;

FIG. 4 is a graphical representation of comparative cycles to failurebased on a Taber Wear Test, detailed in the Examples;

FIG. 5 is a graphical representation of comparative cycles for percentchange in haze based on a Wiper Test, detailed in the Examples;

FIG. 6 is a schematic representation of apparatus used to test flexingin the Examples;

FIG. 7 is a graphic representation of load application to samples usedto test flexing, as detailed in the Examples;

FIG. 8 is a graphical representation of comparative percent change inadhesion index based on a Dry Adhesion Test, detailed in the Examples;and

FIG. 9 is a graphical representation of comparative percent change inadhesion index based on a Wet Adhesion Test, detailed in the Examples.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the described embodiments or the application anduses of the described embodiments. Furthermore, there is no intention tobe bound by any expressed or implied theory presented in the precedingtechnical field, background, brief summary or the following

FIG. 1 illustrates an example of an embodiment of a barrier-coatedpolymeric substrate 100 that includes a polymeric substrate 110. In thisparticular embodiment, the polymeric substrate 110 includes a stretchedacrylic windshield portion, and is therefore optically transparent inthe wavelengths of human vision. A transparent multi-layer barriercoating 150 is applied on the surface 112 of the polymeric substrate110. The transparent barrier coating 150 in this instance includes twotransparent coating components: an underlying multi-layered coating 120and a top indicator coating 130. The top indicator coating presents anouter coating surface 132 to the surrounding environment. Top indicatorcoating 130 may be selected for specific measurable physical propertiesof the outer coating surface 132. For example, in one exemplaryembodiment the outer coating surface 132 may be hydrophobic so that awater droplet 140 on the surface 132 has a contact angle α of about 60°,or greater, as illustrated. The multi-layered coating component 120includes multiple stacked pairs of layers (pairs of layers not shown)that are each formed and chemically bonded to each other to form theunitary multi-layered coating 120. Each pair of the multiple stackedpairs of layers includes a first layer and a second layer, as describedin more detail below. The lowermost, or first formed, of the stackedpairs of layers adheres tightly to the outer surface 112 of thepolymeric substrate 110.

As explained in more detail here below, examples of embodiments ofbarrier-coatings may have any one or more of the followingcharacteristics: good adhesion to a polymeric substrate, excellent wearresistance as measured in applicable tests, no significant UV-induceddegradation due to added UV inhibitors or otherwise, and good elongationunder strain. In aircraft or other windshield applications, the barriercoatings should be optically transparent and have resistance to crazingwhen exposed to chemicals commonly used in the processes of cleaning andmaintaining the vehicle, and conditions ordinarily expected to beencountered during operations. These include, for example, de-icingfluid, jet fuel, methyl ethyl ketone, solvent cleaner, alkaline cleaner,hydraulic fluid and detergent, among others. In addition, exemplaryembodiments may include an indicator useful for determining barriercoating condition.

Barrier-coated polymeric substrates, whether transparent or not, may beuseful in a wide range of industries and applications, for example,passenger aircraft flight deck windshields, automotive windshields andother “plastic” automobile components such as body panels, as well asother durable consumer goods such as LCD or plasma TV screens,architectural or marine windows, and even medical devices such as heartvalve seats. In many instances, barrier-coated transparent polymericsubstrates such as stretched acrylic or polycarbonate may be used as asubstitute for glass. Such substitution may result not only in weightsavings and reduced fragility, but may also result in better performancedepending upon conditions of use. As detailed here below, examples ofembodiments of barrier-coated polymeric substrates outperform glass inseveral physical tests.

In general plasma-based hard coatings exhibit low (about 1.0%) strain tofailure while softer polysiloxane coatings exhibit strain of over 2%.Thus, while the plasma-based hard coatings provide durability, bythemselves they lack the flexibility required for application toaircraft windows. Our prior application, “Durable Transparent Coatingsfor Aircraft Passenger Windows,” U.S. Ser. No. 11/289,920 filed11-30-2005 is hereby fully incorporated by reference, to the extentrelevant about coatings formed with a pair of coating layers. However,it has now been found that barrier coatings of acceptable hardness andductility are achieved for aircraft windshield applications by stackingmultiple pairs of hard and soft coating layers.

Exemplary embodiments of barrier coatings include more than one pair oflayers. A first layer of the pair may include a coupling agent thatadheres strongly to a polymeric substrate while also having functionalmoieties capable of bonding chemically with a second layer which will beformed over the first layer. The coupling agent may includepolysiloxane, hybrid polymers of alkoxysilanes, and the like.Polysiloxanes are common organo-silicon polymers and the silane Si(Me)₂is a key functional group in the synthesis of these polysiloxanes. Ingeneral, coating thickness depends on a variety of factors including,for example, the application process and the nature of the coating layerchemical composition. In exemplary embodiments, the thickness of thefirst applied coating layer may range from about 4 to about 5micrometers, although thinner or thicker coatings are also useful.

A second layer of the coating may include a hard layer. Such a hardlayer may be deposited by plasma techniques, such as ion beam-assistedplasma vapor deposition or plasma-enhanced chemical vapor deposition.Ion bombardment effects during deposition of silicon-based transparentcoatings have been shown to improve the hardness and durability of thecoatings. The second plasma-deposited coating layer bonds to the firstcoating layer chemically and provides a hard outer surface. Examples ofhard, plasma-formed layers that may be used as the second layer include,but are not limited to, DIAMONDSHIELD™ (obtainable from Morgan AdvancedCeramics of Allentown, Pa.); transparent DYLAN™ coatings (BekaertAdvanced Coating Technologies Amherst, N.Y.); and the like. In someembodiments, the second layer may include silica, alumina, zirconia,ceria and like hard oxides. In exemplary embodiments, the second coatingmay be of the formulae:

SiO_(x)C_(y)

where x ranges from 1.0 to 1.2 and y ranges from 1.0 to 0.8

A combination of the first (softer) and second (harder) layer as a pairof coating layers is better able to resist flexing and strain than thehard layer by itself while the hard layer protects the softer fromabrasion. In addition, stacked multiple pairs of layers providessignificant performance improvement over a single pair of layers.Accordingly, exemplary embodiments include multiple pairs of coatinglayers stacked onto a first pair. The first pair has a first layertightly adhered to the polymeric substrate surface.

In addition, exemplary embodiments may include an indicator layerapplied to the uppermost (second) layer of the final pair of coatinglayers, or between coating layers, as may be required. For example theindicator layer may be formed between the final pair of coating layersso that it is protected by the outermost plasma-deposited hard coatinglayer. This indicator layer may be of a type that is readily tested forcoating condition, such as physical wear from abrasion or chemicalattack. If this indicator layer shows signs of wear that signalpotential exposure of coating layers to the environment, thenappropriate remedial action may be taken. In one embodiment, theindicator layer includes a hydrophobic layer and the degree ofhydrophobicity of areas of this indicator layer may be measured todeduce the extent of wear-through of the barrier coating, if any, or anyimminent risk of exposure of the underlying polymeric substrate to harshenvironments or chemicals that might cause damage. In anotherembodiment, a metallic coating may be used. The electrical properties ofthis metallic layer may be correlated to coating condition. Testing anyof these properties, such as resistance or conductivity, may then beused to determine barrier coating condition and whether remedial actionis required.

An example of a method of making barrier-coated polymer substrates 200is shown in FIG. 2. Of course other method embodiments may have fewer ormore steps. In FIG. 2, for example, a transparent stretched acrylicaircraft windshield may be selected as a polymer substrate in process210. The stretched acrylic aircraft windshield may then be subjected tosurface preparation in process 220. Surface preparation may include anysuitable surface treatment based on the nature of the substrate and thesurface condition. In general, for a stretched acrylic aircraftwindshield, detergent and solvent cleaning as well as surface treatmentwith oxygen plasma may be used. After surface preparation in process220, the cleaned, stretched acrylic windshields may be coated with asilane, such as a polysiloxane-based transparent coating composition, inprocess 230, to produce a first coating layer on the windshield. Whenthe silane coating layer has hardened, the silane-coated surface may betreated in process 240 to remove contaminants, such as hydrocarbons andother undesirable materials. Process 240 may include any suitablesurface cleaning process, such as ultrasonic cleaning in solvents, orcleaning with aqueous detergents and/or other chemicals to prepare thesurface for plasma deposition.

A second coating layer may be applied in process 250 using ionbeam-assisted plasma techniques, for example, such as the plasma processused to manufacture Diamondshield™ that uses silicon-containingprecursors, such as hexamethyl disiloxane and oxygen. The plasmadeposition conditions, such as gas flow, deposition pressure, plasmapower etc., are optimized to produce hard, transparent coatings. Whendesired vacuum conditions are obtained, the substrates may first besputter cleaned using inert ions and or oxygen ions. This may then befollowed by plasma deposition of the hard coat. In decision process 260,a check is conducted to see whether the desired number of multiples ofpairs of coating layers has been achieved or not. If additional pairs ofcoatings are needed, the method returns to process step 220, as shown,and repeats processes 220 to 260 until the desired multiples of coatingpairs have been applied. Then an indicator may be applied in process270.

In another example of an embodiment of a process for producing multiplelayered barrier coatings, electron beam deposition techniques may bevaried to deposit pairs of coating layers that each has a hard and asofter layer. In this scheme, direct electron beam deposition produces afirst, softer coating layer and ion assisted deposition creates asecond, harder coating layer. Alternating between direct electron beamdeposition and ion assisted processes creates a sequence of hard andsofter coating layers. Layer thicknesses may be in the nanometer sizerange. Thus, a single layer may be from about 3 to about 10 nanometersthick so that nanostructure-induced (for example, Hall-Petch effect)mechanical strength enhancement effects may be obtained. According tothe Hall-Petch relationship, mechanical strength increases as the grainsize of the material decreases. This relationship holds until the grainsize is the same as the equilibrium distance between dislocations in thegrain structure.

In another example of an embodiment of a process for producing multiplelayered barrier coatings, alternating softer and hard transparentsilicon dioxide coating layers may be formed using Plasma EnhancedChemical Vapor Deposition (PECVD) in conjunction with Electron CyclotronResonance (ECR) sources. The use of high density oxygen plasma generatedwith the in-situ ECR source reduces oxide layer growth temperature tothe range from about 50° C. to about 80° C. and hence makes thistechnique more suitable for polymeric substrates such as acrylic andpolycarbonate. The mechanical characteristics (modulus, hardness) of thedeposited oxide film can be changed by varying substrate bias voltage.Accordingly, this technique may be useful to deposit alternating hardand soft coating layers in pairs to yield a multi-layer barrier-coatedsubstrate.

In exemplary embodiments of a barrier-coated aircraft windshield, anindicator layer, such as a hydrophobic coating, is applied on top of themulti-layer barrier coating. In ordinary use, as the coated windshieldis exposed to the elements and to chemicals used in aircraft cleaningand maintenance, the hydrophobic coating, which is the outermostcoating, may be expected to wear out first. The wearing of thishydrophobic layer and concomitant loss of hydrophobicity at the surfaceindicate that the underling multi-layered coating is exposed. At thatpoint, new hydrophobic coating can be applied. This will preserve theunderlying multi-layer and maintain visibility characteristics of theaircraft windshield.

In another embodiment, the barrier coating may include an indicatorlayer that includes a metallic film sputtered onto the top of themultilayered coating or deposited between pairs of coating layers toprovide a conductive surface. A change in electrical conductivity orother measure of electrical properties of this conductive surface wouldindicate a level of wear in the multilayer coat and this may becorrelated with degradation of its capability to protect the substratepolymeric material. At that point, remedial action may be taken. If themetallic indicator coating is sputtered onto the multi-layer coatingouter surface, the sputter coating may be renewed. If the metallicindicator coating is located between pairs of coating layers to detectwhen wear has reached the indicator layer depth in the coating, then thecoated substrate should be taken out of service.

In an example of an embodiment of a substrate with a barrier coatingthat had six alternating soft and hard layers, when the coating issubjected to a sand erosion test in accordance with ASTM D968-93, thecoated substrate suffered a haze change of not more than about 15%,using about 5 liters of sand of mean size 800 microns.

In another example of an embodiment of a substrate with a barriercoating that had six alternating soft and hard layers, the coatedsubstrate demonstrated resistance to abrasive wear After exposure of thebarrier coating to chemicals selected from de-icing fluid, jet fuel,methyl ethyl ketone, solvent cleaner, alkaline cleaner, hydraulic fluidand detergent, the coated substrate was tested in accordance with TaberWear Test ASTM D-1044-99 for 100 cycles. This resulted in a change inhaze of not more than about 1%.

In another example of an embodiment of a substrate with a barriercoating that had six alternating soft and hard layers, when the barriercoating is subjected to a wiper abrasion durability test in accordancewith D6-82942-1, it suffers a haze change of not more than about 1%after 155,000 cycles.

In another example of an embodiment of a substrate with a barriercoating that had six alternating soft and hard layers, when subjected toa three-point flex test in accordance with a modified ASTM D-790, thesubstrate showed no cracking or crazing after 500 cycles. Modificationsto the ASTM test are shown in the Flex Test example, here below.

EXAMPLES

The following examples reflect tests conducted to compare some of therelevant physical properties of barrier-coated stretched acrylic,stretched acrylic coated with siloxane only, stretched acrylic coatedwith a hard plasma coating only, uncoated stretched acrylic, and glass.

To conduct these comparative tests, stretched acrylic samples of Group Iwere coated with a four micron thick polysiloxane coating. Stretchedacrylic substrates of Group II were coated with a hard plasma coating,and stretched acrylic substrates of Group III were coated with amulti-layer that included three pairs of coating layers.

Sand Erosion Test:

Samples of the coated substrates (Group I, Group II & Group III) alongwith bare glass and bare acrylic substrates were tested to determine theeffect of sand erosion on optical haze. The tests were carried out inaccordance with the procedure described in ASTM D968-93, “Standard Testmethods for Abrasion Resistance of Organic Coatings by FallingAbrasive”. The volume of sand used in each of these tests was fiveliters. The sand used in these tests had a mean diameter of 800 microns.The increase in haze was used as the criterion for measuring theseverity of erosion. The results are summarized in FIG. 3. Thebarrier-coated sample showed the least erosion-damage. The resultinghaze (15%) for this barrier-coated sample was less than half of that fora glass sample (37%).

Application of a hydrophobic coating on to sand abraded surface tends todecrease the haze further for acrylic samples. For example, hazedecreased from 15% to 7% for the acrylic sample with multilayer coatingafter a hydrophobic coating was applied.

Taber Wear Test:

Samples were tested for wear in accordance with the procedure describedin ASTM D-1044-90, “Standard Test Method for Resistance of TransparentPlastics to Surface Abrasion”. This test utilizes two CS-10F wheels thateach bears a load of 500 g. The samples are laid on a table and thewheels abrade the substrate surface as the wheels rotate in a circleover the sample surfaces. Increase in haze is used as the criterion formeasuring the severity of abrasion. In these tests, the samples were runand wheel rotations were counted until the haze had increased by 5% as aresult of abrasion. The results are shown in FIG. 4. The barrier-coatedsample showed excellent gains in wear resistance when compared to apolysiloxane or other hard coating.

Wiper Abrasion Tests:

Coating durability tests were carried out on three groups of acryliccoupons with (1) polysiloxane and (2) a hard coat and (3) a multilayercoating in accordance with D6-82942-1, “Hydrophobic Window Coating TestProtocol.” In these tests, a linear motion test apparatus that includessegments of cockpit window wiper blade in a carriage was used with anautomated drive system equipped with a linear counter. The wiper bladeassembly had a linear load of 0.5 lb/inch (8.937 g/mm) and wiped backand forth on the coated sample. The surface of the test sample was wetwith water containing 400 mesh silicon dioxide abrasive (equivalent tomedium grade Arizona road dust) to accelerate the wear of the coating.Coating durability was evaluated by measuring the number of wiper bladeexcursions necessary to cause a 1% change in haze of the sample. Theresults are summarized in FIG. 5. While the polysiloxane coating lastedonly 13,000 cycles, the multi-layer barrier-coated samples required155,000 cycles to cause 1% haze change.

Flex Test:

A modified ASTM D-790 test protocol was used in conducting the flextests of coated samples. Samples of dimensions 1″×12″×0.5″ with coatings(Group I & II) were subjected to a three point bend test as shown inFIG. 6. Each sample 600 rested on supports 610 placed about 1 inch fromeach end of the sample. A mass 650 applied a load P in the centers ofthe samples 600. The hard coating side 620 faces downwards in thisfigure. A thin film of 75 wt. % sulfuric acid in water was applied tothe coating 620 using a fiber glass filter and a Kapton tape. The testsamples 600 were subjected to a cyclic load/temperature profile shown inFIG. 7. The temperature was repeatedly cycled between −80° F. and 130°F. and held constant at each of these the temperature limits for 10minutes. As seen in FIG. 7, the samples started at 70 ° F. and werecooled to −80° F. over a period of about 15 minutes. The samples werethen held at −80° F. for 10 minutes and reheated to 130° F. over aperiod of about 20 minutes. The samples were held at 130° F. for 10minutes, then cooled to −80° F. over about 20 minutes. The cycle thenrepeated between −80° F. and 130 ° F. The load P was applied cyclicallyas well. Load P was zero initially and increased P=3,600 psi whentemperature reached −80° F. Load was maintained at this level, anddecreased to zero as the temperature increased and reached 70° F. Loadincreased from zero as the temperature decreased from 130 and reached3,600 psi at 70° F. This load was maintained during the 10 minute dwellperiod at −80° F. and decreased in the next heating cycle, to reach zeroat 70° F. The tests were continued until the coating cracked or thesurface exhibited crazing (whichever occurred first). The results showedthat while the polysiloxane coatings failed in 50 cycles, the multilayerbarrier coatings showed no cracking or crazing even after 500 cycles.

Chemical Exposure Test:

Samples of stretched acrylic substrates with polysiloxane (Group I) andmultilayer barrier coatings (Group II) were exposed to chemicals thatare normally used in aircraft maintenance. The samples were exposed toeach chemical for a period of 24 hours (exception: exposure to MEK wasfor 4 hours) and then tested for adhesion (modified ASTM D 3330-BSS7225) type I and III (dry and wet)) and % haze change due to wear (ASTMD-1044-90). The samples with multi-layer coatings exhibited nodegradation in dry adhesion (as indicated by adhesion index) as shown inFIG. 8 or wet adhesion (as indicated by adhesion index) as shown in FIG.9.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of thedescribed embodiments in any way. Rather, the foregoing detaileddescription will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments.It should be understood that various changes can be made in the functionand arrangement of elements without departing from the scope as setforth in the appended claims and the legal equivalents thereof.

1. A coated polymeric substrate comprising: a polymeric substrate bodyhaving a first surface; a coating on the first surface, the coatingcomprising: more than one pair of coating layers, each pair comprising afirst applied coating layer and a second applied coating layer.
 2. Thesubstrate of claim 1, wherein the first applied coating layer comprisespolysiloxane.
 3. The substrate of claim 1, wherein the second appliedcoating layer comprises a plasma enhanced chemical vapordeposition-applied coating layer or an electron beam deposited coatinglayer, the second applied coating layer harder than the first appliedcoating layer.
 4. The substrate of claim 1, wherein the second appliedcoating layer comprises SiO_(x)C_(y) where x ranges from 1.0 to 1.2 andy ranges from 1.0 to 0.8.
 5. The substrate of claim 1, wherein thepolymeric substrate body and the coating are both optically transparentin light of a wavelength region of human vision.
 6. The substrate ofclaim 5, wherein the coating on the first surface of the polymericsubstrate body, when subjected to a wiper abrasion durability test inaccordance with D6-82942-1, suffers a haze change of not more than about1% after 155,000 cycles.
 7. The substrate of claim 5, wherein thecoating on the first surface of the polymeric substrate body, whensubjected to a Taber Wear Test in accordance with ASTM 1044-05, suffersa haze change of not more than about 5% after 40,000 cycles.
 8. Thesubstrate of claim 5, wherein the coating on the first surface of thepolymeric substrate body, when subjected to a sand erosion test inaccordance with ASTM D968-93, suffers a haze change of not more thanabout 15%, using about 5 liters of sand of mean size
 800. 9. Thesubstrate of claim 5, wherein the polymeric substrate body comprises anaircraft windshield comprising stretched acrylic or polycarbonate. 10.The substrate of claim 4, wherein thickness of the first applied coatinglayer is in a range from about 4 to about 5 micrometers.
 11. Thesubstrate of claim 1, further comprising an indicator applied on top ofor between coating layers, the indicator providing indicia of wear ofthe coating.
 12. The substrate of claim 11, wherein the indicatorcomprises a metallic coating layer or a hydrophobic coating layer
 13. Apolymeric substrate comprising: a polymeric substrate body configured ina size and shape of an aircraft windshield, the polymeric substrate bodyhaving a first surface corresponding to an outer surface of the aircraftwindshield; a coating on the first surface, the coating comprising: morethan one pair of coating layers, each pair comprising a first appliedcoating layer and a second applied coating layer; and an indicatorapplied on top of or between coating layers, the indicator providingindicia of wear of the coating; wherein, after exposure of the coatingto chemicals selected from de-icing fluid, jet fuel, methyl ethylketone, solvent cleaner, alkaline cleaner, hydraulic fluid, anddetergent, testing in accordance with a Taber Wear Test ASTM D-1044-99for 100 cycles results in a change in haze of not more than about 1%.14. The substrate of claim 13, wherein the first applied coating layercomprises polysiloxane.
 15. The substrate of claim 13, wherein thesecond applied coating layer comprises a plasma enhanced chemical vapordeposition-applied coating layer or an electron beam deposited coatinglayer, the second applied coating layer harder than the first appliedcoating layer.
 16. The substrate of claim 13, wherein the second appliedcoating layer comprises SiO_(x)C_(y) where x ranges from 1.0 to 1.2 andy ranges from 1.0 to 0.8.
 17. The substrate of claim 13, wherein thecoating, when subjected to a wiper abrasion durability test inaccordance with D6-82942-1, suffers a haze change of not more than about1% after 155,000 cycles.
 18. The substrate of claim 5, wherein thecoating, when subjected to a sand erosion test in accordance with ASTMD968-93, suffers a haze change of not more than about 15%, using about 5liters of sand of mean size 800 microns.
 19. A method of testing anaircraft windshield comprising: examining the aircraft windshieldcomprising a polymeric composition, the aircraft windshield having acoating on an external surface thereof, the coating comprising: morethan one pair of coating layers, each comprising a first applied coatinglayer and a second applied coating layer; an indicator applied on top ofor between coating layers, the indicator providing indicia of wear ofthe coating; and determining, based on the indicator, whether theaircraft windshield requires maintenance, replacement or repair.
 20. Themethod of claim 19, wherein the indicator comprises a hydrophobiccoating.
 21. The method of claim 19, wherein the indicator comprises ametallic coating.